The present invention relates to radio communications technology and more particularly to direct conversion radio receivers.
At the present time the vast majority of radio receivers are of the superheterodyne type employing one or more intermediate frequency stages which allow for filtering and amplification at fixed frequencies. Alternatives have always existed to the superheterodyne architecture such as superregenerative and direct conversion designs. However, these alternative designs have been subject to serious flaws which have relegated radio receivers of these types to specialty roles within the radio communications world.
Despite the widespread adoption of the superheterodyne design, it has been widely recognized that the direct conversion architecture holds great promise for superior performance. For example, direct conversion receivers are not subject to image rejection problems and are not affected by cross-over spurious responses which are so often the cause of interference and related design difficulties in superheterodyne receivers. Further, direct conversion receivers feature simpler low-pass type filters operating at audio frequencies in contrast to the often bulky and expensive bandpass filters employed in superheterodyne receivers, require only a single injection signal at one frequency rather than multiple signals at different frequencies (multiple conversion sets), and provide a good potential for VLSI implementations since a majority of the receiver components consist of active audio and digital circuitry.
In a typical I/Q direct conversion receiver incoming RF communications signals are split into a pair of equal amplitude components which are in phase with each other. These RF components are then mixed with separate injection signals at approximately the same frequency as the communications signal but which are 90.degree. out-of-phase with each other. I and Q baseband signal components which are in quadrature are thereby generated. These signals are then independently filtered and amplified at audio frequencies on separate signal channels. The I and Q components formed as a result of the mixing process allow the signal to be conveniently demodulated upon being supplied to a suitable signal processing unit.
This architecture works well except that it is very difficult to achieve and maintain identical gain and exact phase quadrature between the signal channels and variations between the signal channels which commonly occur as a result of changes in temperature, frequency and other operational parameters result in gain and phase mismatches with produce distortion products in the output of the receiver. Gain mismatches of as little as 0.2 dB and phase mismatches of as little as 1.degree. can result in distortion products which can not ordinarily be reduced to less than 30-40 dB in practice and correspond to discrete tones which greatly limit the performance of the receiver.
Researchers investigating the design of direct conversion radio receivers have frequently recognized this limitation and a number of systems for correcting for errors between quadrature signal channels have been proposed. However, these systems have generally been specialized designs limited to the processing of signals of only a single modulation type. For example, U.S. Pat. No. 4,926,433 to Werner Reich entitled "Correction Circuit For A Digital Quadrature-Signal Pair" describes a correction system including an error-detecting stage for deriving amplitude, offset and phase errors from which correction signals are formed. However, error detection is limited to wideband FM signals characterized by quadrature signal pairs capable of forming an "elliptical locus" from which the errors can be determined by comparison with an ideal circle. In contrast, AM signals result in such a locus taking on irregular shapes from which proper error signals cannot be derived.
The publication by Bolliger and Vollenweider entitled "Some Experiments on Direct Conversion Receivers" in the proceedings of the Fifth International Conference on Radio Receivers and Associated Systems held Jul. 24-26, 1990 at Oxford, England describes a useful method for reducing the gain and phase errors between signal channels in an I/Q direct conversion receiver. In accordance with this method, new signals I' and Q' are formed which are equal to Q.sup.1 -I.sup.2 and 2IQ where I and Q represent the original baseband components. If the expressions Q.sup.2 -I.sup.2 and 2IQ are mathematically expanded, phase and gain errors may be seen to be primarily resident in DC terms which can be removed from the signals in order to reduce distortion. Accordingly, the I' and Q' signals are highpass filtered to suppress their DC components in order to produce derivative signals for direct use in demodulation which are characterized by reduced levels of gain and above errors.
The rationale for forming these signals may be readily understood with reference to the basic trigonometric identities cos(2.theta.)=cos.sup.2 (.theta.) -sin.sup.2 (.theta.) and sin(2.theta.)=2 cos(.theta.) sin(.theta.). Since the signals I and Q are in quadrature, they may be seen to correspond to cos(.theta.) and sin(.theta.). Consequently, the expressions Q.sup.2 -I.sup.2 and 2IQ are related to 2.theta. which allows the original phase angle .theta. to be determined based on the signals I' and Q'.
This method of reducing distortion due to gain and phase errors between signal channels is highly useful but is limited to angle modulated signals since amplitude modulation would result in the DC terms being converted to low frequency AC components which would be resistant to filtering. Furthermore, this method does not provide for detection of the actual levels of the phase and gain errors and full and accurate correction for such errors on a continuing basis.
It is, therefore, an object of the present invention to provide an I/Q direct conversion radio receiver having a pair of signal channels carrying I and Q baseband signal components which are in quadrature which is characterized by superior performance due to the absence of distortion products arising from gain and phase errors between the signal channels.
It is another object of the present invention to provide a system of ruse in an I/Q direct conversion radio receiver which is adapted for processing signals of all modulation types and automatically detecting phase and gain errors resulting from mismatches between the signal channels within the receiver and fully correcting for such errors pursuant to a straightforward signal processing algorithm.
It is a yet further object of the present invention to provide a new system for controlling phase and gain errors in an I/Q direct conversion radio receiver which does not require specially generated calibration signals, is economic to construct, provides superior performance and may be substantially implemented in VLSI.